Anode structure

ABSTRACT

An anode structure comprising a ruthenium catalyst is disclosed. The catalyst consists essentially of ruthenium deposited on a conducting support wherein the ruthenium is in metallic form or in a form that is readily reducible to the metallic form at temperatures of 25° C. to 150° C. The anode structure is particularly of use in proton exchange membrane fuel cell to prevent poisoning of the electrocatalyst by impurities in the fuel stream to the anode.

This invention relates to a novel anode structure, particularly suitablefor use in proton exchange membrane fuel cells, which may be used forthe removal of impurities from an impure gas stream, particularly forthe removal of carbon monoxide from a reformate fuel stream.

Electrochemical cells invariably comprise at their fundamental level asolid or liquid electrolyte and two electrodes, the anode and cathode,at which the desired electrochemical reactions take place. A fuel cellis an energy conversion device that efficiently converts the storedenergy of its fuel into electrical energy by combining hydrogen, storedas a gas, or methanol, stored as a liquid or gas, with oxygen togenerate electrical power. The hydrogen or methanol is oxidised at theanode and oxygen reduced at the cathode. In these cells gaseousreactants and/or products have to be diffused into and/or out of thecell electrode structures. The electrodes therefore are specificallydesigned to be porous to gas diffusion in order to optimise the contactbetween the reactants and the reaction sites in the electrode tomaximise the reaction rate. An electrolyte is required which is incontact with both electrodes and which may be alkaline or acidic, liquidor solid. In a solid polymer fuel cell (SPFC), also known as aproton-exchange membrane fuel cell (PEMFC), the electrolyte is a solidproton-conducting polymer membrane, commonly based on perfluorosulphonicacid materials. These electrolytes must be maintained in a hydrated formduring operation in order to prevent loss of ionic conduction throughthe electrolyte; this limits the operating temperature of the PEMC tobetween 70° C. and 120° C., depending on the operating pressure. ThePEMFC does, however, provide much higher power density output than theother fuel cell types, and can operate efficiently at much lowertemperatures. Because of this, it is envisaged that the PEMFC will finduse in vehicular power generation and small-scale residential powergeneration applications. In particular, vehicle zero-emissionregulations have been passed in areas of the United States that arelikely to restrict the use of the combustion engine in the future.Pre-commercial PEMFC-powered buses and prototype PEMFC-powered vehiclesare now being demonstrated for these applications.

Due to the relatively low operating temperatures of these systems, theoxidation and reduction reactions require the use of catalysts in orderto proceed at useful rates. Catalysts, which promote the rates ofelectrochemical reactions, such as oxygen reduction and hydrogenoxidation in a fuel cell, are often referred to as electrocatalysts.Precious metals, in particular platinum, have been found to be the mostefficient and stable electrocatalysts for all low-temperature fuel cellsoperating below 300° C. The platinum electrocatalyst is provided as verysmall particles (˜20–50 Å) of high surface area, which are often, butnot always, distributed on and supported by larger macroscopicconducting carbon particles to provide a desired catalyst loading.Conducting carbons are the preferred materials to support the catalyst.

In the PEMFC the combined laminate structure formed from the membraneand the two electrodes is known as a membrane electrode assembly (MEA).The MEA will typically comprise several layers, but can in general beconsidered, at its basic level, to have five layers, which are definedprincipally by their function. On either side of the membrane an anodeand cathode electrocatalyst is incorporated to increase the rates of thedesired electrode reactions. In contact with the electrocatalystcontaining layers, on the opposite face to that in contact with themembrane, are the anode and cathode gas diffusion substrate layers. Theanode gas diffusion substrate is designed to be porous and to allow thereactant hydrogen or methanol to enter from the face of the substrateexposed to the reactant fuel supply, and then to diffuse through thethickness of the substrate to the layer which contains theelectrocatalyst, usually platinum metal based, to maximise theelectrochemical oxidation of hydrogen or methanol. The anodeelectrocatalyst layer is also designed to comprise some level of theproton conducting electrolyte in contact with the same electrocatalystreaction sites. With acidic electrolyte types the product of the anodereaction are protons and these can then be efficiently transported fromthe anode reaction sites through the electrolyte to the cathode layers.The cathode gas diffusion substrate is also designed to be porous and toallow oxygen or air to enter the substrate and diffuse through to theelectrocatalyst layer reaction sites. The cathode electrocatalystcombines the protons with oxygen to produce water. Product water thenhas to diffuse out of the cathode structure. The structure of thecathode has to be designed such that it enables the efficient removal ofthe product water. If water builds up in the cathode, it becomes moredifficult for the reactant oxygen to diffuse to the reaction sites, andthus the performance of the fuel cell decreases.

The complete MEA can be constructed by several methods. Theelectrocatalyst layers can be bonded to one surface of the gas diffusionsubstrates to form what is known as a gas diffusion electrode. The MEAis then formed by combining two gas diffusion electrodes with the solidproton-conducting membrane. Alternatively, the MEA may be formed fromtwo porous gas diffusion substrates and a solid proton-conductingpolymer membrane catalysed on both sides; or indeed the MEA may beformed from one gas diffusion electrode and one gas diffusion substrateand a solid proton-conducting polymer membrane catalysed on the sidefacing the gas diffusion substrate.

Gas diffusion substrates or electrodes are employed in many differentelectrochemical devices in addition to fuel cells, including metal-airbatteries, electrochemical gas sensors, and electrochemical reactors forthe electrosynthesis of useful chemical compounds.

In most practical fuel cell systems, the hydrogen fuel is produced byconverting a hydrocarbon-based fuel (such as methane) or an oxygenatedhydrocarbon fuel (such as methanol) to hydrogen in a process known asreforming. This fuel, referred to as reformate, contains (in addition tohydrogen) small amounts of impurities such as carbon monoxide (CO),typically at levels of around 1%. For fuel cells operating attemperatures below 200° C., and especially for the PEMFC operating attemperatures around 100° C., it is well known that CO, even at levels of1–10 ppm, is a severe poison for the platinum electrocatalysts presentin the electrodes. This leads to a significant reduction in fuel cellperformance, i.e. the cell voltage at a given current density isreduced. This deleterious effect is more pronounced in PEMFCs operatingat lower temperatures.

Various methods have been employed to alleviate anode CO poisoning. Forexample, reformer technology has been redesigned to include anadditional catalytic reactor, known as a preferential or selectiveoxidation reactor. This involves the injection of air or oxygen into thehydrogen-containing reactant gas stream, prior to it passing over aselective oxidation catalyst, to oxidise the CO to CO₂. This can reducethe levels of CO from 1–2% down to below 100 ppm. However, even at theselevels, the anode electrocatalyst in the PEMFC is still poisoned.

It has also been found that poisoning of the electrocatalyst by CO atlevels of 1–100 ppm can be reduced by the use of an oxygen or air bleeddirectly into the anode gas stream just before it enters the anodechamber of the fuel cell itself. This is described by Gottesfeld andPafford in J. Electrochem. Soc., 135, 2651 et seq. (1988). Thistechnique is believed to have the effect of oxidising the residual CO inthe fuel to CO₂, the reaction being catalysed by electrocatalyst sitespresent in the anode:CO+½ O₂→CO₂

This technique provides fuel cell performance that is much closer to theperformance observed when no CO is present in the fuel stream.

A further technique for alleviating fuel cell performance reduction dueto anode CO poisoning is to employ an anode electrocatalyst which isitself intrinsically more poison tolerant, but which still functions asa hydrogen oxidation catalyst in the presence of CO. With this approachit is not necessary to employ the air bleed technique described above toobtain improved performance. As described by, for example, L Niedrach etal in Electrochemical Technology, Vol. 5, 1967, p 318, the use of abimetallic anode electrocatalyst comprising platinum/ruthenium, ratherthan the more conventionally used mono-metallic platinum onlyelectrocatalyst, shows a reduction in the poisoning effect of the CO attypical PEMFC operating temperatures. The bimetallic catalyst does not,however, reduce the levels of CO in the reactant fuel stream, but isslightly more tolerant towards the presence of CO than platinumelectrocatalyst alone. However, again it has not yet been possible tofully attain the performance observed on pure hydrogen, i.e. in theabsence of CO in the fuel stream, by using this approach in isolation.

It thus appears that there exist two commonly used techniques forimproving the performance of fuel cell anodes for operation on reformatefuel comprising trace levels of CO, i.e. the use of an air bleed and theuse of a more poison tolerant electrocatalyst. However, the improvementthe techniques offer are explained by the operation of two differentreaction mechanisms. Firstly, with the air bleed technique, it ispostulated that in the presence of oxygen the anode electrocatalystfacilitates the oxidation of CO to CO₂, as described in the reactionabove. The low level of CO₂ produced from the CO does not have a majorpoisoning effect. Secondly, even in the absence of air bleed, thepoisoning effect of CO can be reduced by using a modified anodeelectrocatalyst (i.e. one that is more tolerant towards the poison). Themechanism proposed for this improvement is that the active sites on themodified electrocatalyst are less prone to poisoning by adsorption ofthe poisoning species and more sites are left available to perform thedesired hydrogen oxidation reaction.

Currently low temperature fuel cells, such as the PEMFC, typicallyemploy electrodes comprising a single catalyst component to acceleratethe hydrogen oxidation and oxygen reduction reactions. The prior artprovides many examples of this. For example, R Lemons in Journal ofPower Sources, Vol. 9, 1990, p 251, shows that similar single componentplatinum catalysts are used for both anode and cathode reactions inPEMFC technology.

In the case of the PEMFC, operating on reformate fuel containing CO inaddition to hydrogen, this type of electrode does not provide sufficientactivity or durability for practical applications. From a cost point ofview it is desirable to use electrodes with loadings of the preciousmetal electrocatalyst of lower than 1.0 mg/cm² of electrode area. Atthese loadings, it has not yet been possible to produce an anodeelectrocatalyst with high enough intrinsic tolerance to poisoning, suchthat, when no air bleed is employed, the performance is close to thatobserved with hydrogen fuel with no poisoning species present.

The air bleed technique has most frequently been employed in PEMFCs inwhich the anode also comprises a conventional single phaseelectrocatalyst material. This is typically a bimetallicplatinum/ruthenium catalyst. Although it is possible to improve theperformance of the PEMFC to close to the level that would be observed ifno poisoning species were present, there are concerns over the long termsustainability of the performance when this conventional type ofelectrode is employed. This is particularly the case if high levels ofair bleed, equivalent to 4% and above of the total reformate fuelvolume, are required.

A recent approach to minimise the effect of CO poisoning by use of anair bleed is disclosed in U.S. Pat. No. 5,482,680. This patent disclosesthe use of a selective oxidation catalyst, present as a gas-porous bedor layer, placed between the fuel stream inlet of the fuel cell and theanode catalyst layer. In particular, the catalyst bed or layer can beplaced in a variety of positions within the fuel stream manifold,including within the fuel stream inlet and fuel stream humidificationapparatus.

EP 0 736 921 discloses an electrode with a first and a second catalyticcomponent, the first catalytic component being designed to be a gasphase catalyst capable of removing the impurities of an impure gasstream. The preferred gas phase catalyst is disclosed as being platinumsupported on carbon. Although the platinum on carbon described in theabove mentioned application shows an improvement in the concentration ofimpurities in the reformate stream, it would still be advantageous toreduce the impurity concentration even further or to obtain the sameefficiency in the reduction of impurities in the reformate stream usinga lower level of air bleed. In addition, platinum is an expensive metaland it would be advantageous to obtain the same reduction in impurityconcentration but at a lower cost.

To this end the present inventors have discovered that the metallicstate of ruthenium is a more effective gas phase catalyst and willefficiently remove impurities, in particular carbon monoxide from animpure stream. However, ruthenium readily oxidises in air to give RuO₂,which in itself is not an appropriate catalyst. The inventors have thusfound that removal of impurities can efficiently be carried out by usingmetallic ruthenium or ruthenium in a form that is able to be readilyreduced to the metallic state of ruthenium under fuel cell operatingconditions, for example ruthenium oxide. Accordingly, a first aspect ofthe invention provides an anode structure comprising a rutheniumcatalyst, characterised in that said catalyst consists essentially ofruthenium deposited on a conducting support wherein the ruthenium is atleast partially present in metallic form or in a form that is readilyreducible to the metallic form at temperatures of 25° C. to 150° C.

A readily available technique which may be used to determine whether theruthenium is in a state capable of being readily reduced to the metallicform at temperatures of from 25° C. to 150° C. is known at TemperatureProgrammed Reduction (TPR). The TPR technique involves cooling thesample in an inert atmosphere (usually nitrogen) to a temperature belowroom temperature (usually −100° C.).The gas mixture is changed to ˜10%hydrogen in nitrogen and after stabilisation the sample temperature isslowly increased and the output/exhaust gas is analysed. A detectormeasures the levels of hydrogen in the output/exhaust gas; depletion ofhydrogen in the output gas corresponds to uptake of hydrogen by thesample, which equates to reduction of the ruthenium compound to metallicruthenium. By analysing the output gas as the temperature is increased,a TPR profile for the sample is obtained, and the temperature over whichthe species is active is determined.

The anode structure according to the present invention is suitably foruse in proton exchange membrane fuel cells which may operate attemperatures of 25° C. up to 150° C., but suitably operate attemperatures of from 50° C. to 100° C.

As mentioned above, the ruthenium used in the anode structure of theinvention is required to be at least partially present either in themetallic state or in a state which is capable of being reduced to themetallic state at the given temperatures. It has been found thatamorphous or poorly crystalline states or a combination of the two areparticularly preferred. Determination of the particular state of theruthenium can readily be determined by known techniques, such as X-rayDiffraction and Transmission Electron Microscopy.

The ruthenium is deposited on a conducting support, suitably a carbonsupport such as Cabot Vulcan XC72R.

The term anode structure in the context of the present invention meansany of the functional components and structures associated with theanode side of the MEA through which hydrogen or methanol fuel is eithertransported or reacted, i.e. within the gas diffusion substrate andelectrocatalyst containing layers on the anode side of the membrane. Theanode structure of the invention is suitably used in a PEM fuel cellwhen an impure fuel is fed to the anode. The anode structure may be usedwith or without the presence of an air bleed. Suitably, the anodestructure is used in a PEM fuel cell to prevent poisoning of theelectrocatalyst metal on the anode side of the MEA; therefore theruthenium catalyst is suitably positioned in the anode structure at anypoint before the impure gas stream reaches the electrocatalyst metal inthe MEA. Thus, specific embodiments of the invention include:

-   (i) a gas diffusion substrate comprising a ruthenium catalyst,    characterised in that said catalyst consists essentially of    ruthenium deposited on a conducting support wherein the ruthenium is    in metallic form or in a form that is readily reducible to the    metallic form at temperatures of 25° C. to 150° C. The ruthenium    catalyst may be applied to either face of the gas diffusion    substrate (i.e. either facing the gas stream or away from the gas    stream, when in use as part of a PEM fuel cell) or embedded within    the gas diffusion substrate or a combination thereof.-   (ii) a gas diffusion electrode comprising a gas diffusion substrate    coated with a layer of an electrocatalyst and further comprising a    ruthenium catalyst, characterised in that said catalyst consists    essentially of ruthenium deposited on a conducting support wherein    the ruthenium is in metallic form or in a form that is readily    reducible to the metallic form at temperatures of 25° C. to 150° C.    Again the ruthenium catalyst may be applied to either face of the    gas diffusion substrate or embedded within the gas diffusion    substrate or a combination thereof. If the ruthenium catalyst is    applied to the face of the gas diffusion substrate also having    applied thereto the layer of electrocatalyst, then suitably the    ruthenium catalyst is first applied to the substrate and    subsequently the electrocatalyst is applied to the ruthenium    catalyst layer.-   (iii) an electrocatalyst coated membrane comprising a ruthenium    catalyst, characterised in that said catalyst consists essentially    of ruthenium deposited on a conducting support wherein the ruthenium    is in metallic form or in a form that is readily reducible to the    metallic form at temperatures of 25° C. to 150° C. Suitably, the    ruthenium catalyst is applied to the electrocatalyst layer which has    previously been applied to a membrane.

The ruthenium catalyst may be applied to the gas diffusion substrate ormembrane by any technique well known in the art. For example, theruthenium catalyst may first be formulated into an ink composition bycombining the ruthenium catalyst with a polymer, preferably ahydrophobic polymer such as PTFE or FEP, and then applying the inkcomposition to the substrate or membrane by known techniques, such asscreen printing, filter transfer or other means. The substrate ormembrane must be formulated in such as way as to preserve the rutheniumcatalyst in a suitable form to be reduced to ruthenium metal in thepresence of hydrogen at temperatures of 25° C. to 150° C. For example,it is preferable that the presence of hydrogen at temperatures of 25° C.to 150° C. For example, it is preferable that the resulting substrate ormembrane is not subjected to temperatures greater than approximately375° C., preferably 275° C. In the situation where the anode structureis an electrocatalyst coated membrane, it is preferable that thestructure is not subjected to temperatures greater than thedecomposition temperature of the membrane. It is also preferred that anyfiring or heat treatment of the substrate or membrane is carried out inan environment devoid of oxygen, for example it is preferable that anyfiring or heat treatment of the substrate or membrane is carried out innitrogen.

The electrocatalyst may be applied to the gas diffusion substrate ormembrane by any technique well known in the art. Suitableelectrocatalysts include platinum/ruthenium alloy catalysts. Theelectrocatalyst ink suitably comprises a proton-conducting ionomer suchas Nafion®.

In a further aspect the invention provides a membrane electrode assemblycomprising an anode structure according to the invention.

In a yet further aspect the invention provides a fuel cell comprising amembrane electrode assembly according to the invention.

The invention will now be illustrated by Examples which are illustrativeand not limiting of the invention.

Composition Preparation

The platinum catalyst used for Comparative Examples 1 and 2 (20% Ptsupported on XC72R carbon) was prepared as described in EP 0 736 921.The platinum catalyst was then formulated into an ink using PTFE.

The ruthenium catalyst for use in Comparative Example 3 and the threeexamples of the invention was prepared by deposition of ruthenium ontothe conductive carbon black substrate to give a catalyst with 20%ruthenium supported on XC-72R carbon. The catalyst was prepared viahydrolysis of an aqueous solution of ruthenium trichloride by a solutionof sodium hydrogen carbonate in the presence of the carbon black, asdisclosed in EP 0 450 849. The catalyst was filtered, washed free ofsoluble chloride salts and dried in a vacuum oven at 80° C. Theruthenium catalyst was then formulated into an ink using either PTFE orFEP.

Electrode Preparation

Electrodes were prepared by application of the ink comprising theplatinum catalyst (for Comparative Examples 1 and 2) or the inkcomprising the ruthenium catalyst (for Comparative Example 3 andExamples 1 to 3) to pre-teflonated Toray TGP090 paper and firing, eitherin air or in nitrogen as described in EP 0 736 921. Table 1 givesdetails of the Comparative Examples and Examples, the metal/metalloading, the polymer used and the firing conditions.

TABLE 1 Metal/ metal loading Example (mg/cm²) Polymer Firing ConditionsComparative Pt/0.2 PTFE 375° Example 1 18% Air Comparative Pt/0.3 PTFE375° Example 2 18% Air Comparative Ru/0.3 PTFE 375° Example 3 18% AirExample 1 Ru/0.3 PTFE 375° 18% Nitrogen Example 2 Ru/0.3 FEP 275° 12%Nitrogen Example 3 Ru/0.2 FEP 275° 12% Nitrogen

Sample Evaluation

The ruthenium catalyst and examples of the invention underwent thefollowing tests: TPR profile measurements, XRD and/or TEM studies and anex-situ test of catalyst activity to fully define the properties of theactive catalyst layer.

TPR Measurements

The ruthenium catalyst, Comparative Example 3 and Examples 1, 2 and 3were subjected to TPR measurement to determine the catalytic activityprofile at temperatures from −100° C. up to 400° C. The results areshown in FIG. 1.

The TPR profiles for the ruthenium catalyst and Example 1 to 3 showpeaks between 50 and 90° C. demonstrating that the ruthenium is presentin a form that is readily reducible under fuel cell operatingtemperatures. The TPR profile for Comparative Example 3 shows no peaksbetween 25° C. and 150° C. and therefore is not reducible to themetallic form of ruthenium at fuel cell operating temperatures.

Ex-situ Example Evaluation

The gas phase selective oxidation layers of Comparative Examples 1 to 3and Examples 1 to 3 were tested for the removal of CO from a H₂ rich gasstream with the addition of an air bleed, in an experimental set-upwhich was similar to that of a fuel cell. This is termed ex-situevaluation. The selective oxidation layers were fabricated into small(6.45 cm²) MEAs using a bare piece of Toray TGP90 as a ‘cathode’ andeither a piece of Nafion 115 or a piece of 0.1 mm thick photocopiertransparency film, as the membrane. The MEAs were tested in a small fuelcell, with the selective oxidation electrode run as the anode. Ahumidified fuel stream of 100 ppm CO in H₂ was used at a gas flow of 0.2SLM at a pressure of 30 psi. Humidified N₂ at similar flow rates andpressures was used as a ‘cathode’ stream. The effectiveness of theselective oxidation electrode was assessed by introducing differentlevels of air bleed into the fuel stream and monitoring the CO level inthe output gas stream using a Signal 2000 low flow CO analyser. Noelectrical load was applied to the electrodes, but the test set upotherwise mimicked the conditions of temperature, humidity and flowrates present within a fuel cell. The results are given in Table 2.

TABLE 2 CO level after 30 minutes at steady state Example 1% Air bleed2% Air Bleed 5% Air Bleed Comparative Example 1 83 74 4 ComparativeExample 2 75 12 4 Comparative Example 3 100 100 96 Example 1 20 14 10Example 2 40 6 6 Example 3 50 15 9

It can be seen from Table 2 that the Examples 1–3 clearly demonstrate amuch greater ability than the examples of state of the art platinumcatalysts for reducing the levels of carbon monoxide with low levels ofair bleed Furthermore, the ruthenium catalyst used in ComparativeExample 3 has clearly been rendered inactive during the electrodeformation. This result can be further explained by considering the XRDand TEM measurements.

XRD and TEM Measurements

XRD and TEM measurements were carried out on the ruthenium catalyst,Comparative Example 3 and Examples 1 to 3. The results are shown inTable 3.

TABLE 3 XRD Crystallite TEM Sample Main phase size (nm) Main phaseParticle size Ruthenium Ru 3.6 [1] Catalyst Comparative RuO₂ RuO₂needles 10–50 nm Example 3 tetragonal Example 1 RuO₂ and Ru — Example 2— — Ru   5 nm particles clustering to  100 nm Example 3 — — Ru   5 nmparticles clustering to  100 nm [1] The ruthenium catalyst was reducedin flowing 10% H₂/N₂ at 150° C. for 2 hours prior to the XRDmeasurement.

As can be seen from Table 3, the active electrodes are those in whichthe ruthenium is at least partially present in the metallic form. Theruthenium in Comparative Example 3 which shows no activity is present inthe crystalline state as RuO₂ needles. Example 1 shows a mixture ofRu/RuO₂ which suggests that it is not necessary for all of the Rutheniumto be in a reducible state.

Fuel Cell Testing

Fuel cell anodes were prepared as described in EP 0 736 921 comprising aselective oxidation layer and an electrocatalyst layer. Theelectrocatalyst layer consisted of a PtRu alloy catalyst at nominalloading of 40% Pt and 20% Ru supported on Cabot Vulcan XC-72R applied tothe substrate in the form of a Nafion ink. Two anodes comprising aruthenium catalyst selective oxidation layer were prepared. Theruthenium catalyst inks were as used in examples 1 and 2. Two otheranodes were prepared for comparison purposes (i) with no catalyst in theselective oxidation layer and (ii) with a platinum catalyst in theselective oxidation layer (the platinum catalyst ink was as used incomparative example 2). The anode samples are described in Table 4:

TABLE 4 Selective Oxidation Layer Anode 1 Ruthenium catalyst as used inexample 1 (0.3 mg/cm² Ru, 18% PTFE). Anode 2 Ruthenium catalyst as usedin example 2 (0.3 mg/cm² Ru, 12% FEP). Comparative No catalyst.Shawinigan carbon/PTFE layer. Anode 1 Comparative Platinum catalyst asused in comparative example 2 Anode 2 (0.3 mg/cm² Pt, 18% PTFE).

The different anode samples described above were made into membraneelectrodes assemblies (MEAs) using Nafion 115 membranes and conventionalcathodes with nominal Pt loadings of 0.75 mgPt/cm². The MEA was preparedby hot pressing the membrane between the anode and cathode. Testing wascarried out in Ballard Mk V hardware at 30 psig and 80° C. using Nafion117 internal humidifiers and H₂:O₂ stoichiometries of 1.5:2. Sampleswere then conditioned in single cells for 2 days and the performance ona synthetic reformate mixture of the composition 100 ppm CO/H₂ wasrecorded. FIG. 2 shows a graph of the performance of the different MEAs.Initially the performance on pure hydrogen and air (at the cathode) isrecorded. After several minutes the anode gas stream is switched to 100ppm CO in hydrogen, which causes degradation in cell voltage. For eachsample different levels of air bleed, 1, 2 and 5% are applied and theperformance allowed to stabilise at each level.

FIG. 2 clearly shows the CO oxidation layer in the MEAs prepared fromthe anodes 1 and 2 show equal or better performance that a PtRu layeralone or a PtRu+Pt layer. Two other beneficial features of a rutheniumbased material are also illustrated by this graph. Firstly the slowerdecline in MEA performance in the presence of the ruthenium layer whentransferring from hydrogen to a poisoning mixture would suggest that thereduced ruthenium can absorb more than one carbon monoxide molecule.This property of the catalyst layer would reduce the impact on MEAperformance of carbon monoxide spikes generated by a reformer unit.Secondly, both the MEAs using anode structures of the invention show agreater intrinsic tolerance to 100 ppm CO level before the applicationof the air bleed. This may indicate a CO clean up reaction occurringwithin the ruthenium catalyst layer in the absence of air bleed,possibly water gas shift or methanation.

Durability Testing

A durability study was carried out on three different MEAs consisting of1000 hours of testing, using real reformate generated by a Methanol HotSpot reformer and Demonox unit as anode fuel. The MEAs comprised anodescorresponding to anode 2, comparative anode 1 and comparative anode 2,as described above. Typically the anode reformate fuel used duringcontained 52% H₂, 27% N₂, 21% CO₂ and 40 ppm CO and a 2% air bleed wasapplied throughout the durability study. Diagnostic tests were carriedout at the start of life and after 500 and 1000 hours to investigatedurability and the effect of long term use of air bleed on MEAperformance.

Table 5 shows performance losses for the three MEA using a 40 ppmCO/25%CO₂/H₂ fuel mix on the anode and the effect of addition of differentlevels of air bleed. This anode poisoning test was carried out atdifferent stages of the durability study to assess deterioration of thesamples

TABLE 5 Performance loss in mV, 40 ppm CO/25% CO₂/H₂ anode gas streamMEA 0% air bleed 1% air bleed 2% air bleed 5% air bleed MEA comprisingComparative Anode 1 Start 111 47 20 17  500 hrs 118 67 42 24 MEAcomprising Comparative Anode 2 Start 110 64 22 16  500 hrs 134 35 22 201000 hrs 129 42 21 19 MEA Comprising Anode 2 Start 113 27 13 12  500 hrs115 19 13 11 1000 hrs 122 23 10 8

The performance losses with 1–5% air bleed for the MEA comprising Anode2 are seller than those comprising Comparative Anodes 1 and 2,indicating the use of a ruthenium catalyst in a selective oxidationlayer can enhance air bleed response.

Furthermore, the good performance of the MEA comprising Anode 2 ismaintained after 1000 hours on reformate and air bleed indicating thedurability of the layer.

At all stages of testing the MEA comprising Anode 2 requires less airbleed for the same level of recovery, indicating the Ru catalyst is moreactive than the Pt/PtRu anode and PtRu anode examples.

1. An anode structure comprising a proton exchange membrane, anelectrocatalyst layer thereon, and a layer comprising a rutheniumcatalyst on a surface of the electrocatalyst layer opposite the protonexchange membrane or separated from the electrocatalyst layer by aninterspersed gas diffusion substrate, or a combination of these; whereinsaid ruthenium catalyst consists essentially of ruthenium deposited on aconducting support wherein the ruthenium is at least partially presentin metallic form or in a form that is readily reducible to the metallicform at temperatures of 25° C. to 150° C., and wherein the rutheniumcatalyst and the electrocatalyst are arranged such that a reactant gasstream will first contact the ruthenium catalyst and thereafter contactthe electrocatalyst.
 2. The anode structure according to claim 1,wherein the ruthenium catalyst is a gas phase catalyst.
 3. The anodestructure according to claim 1, wherein the layer comprising theruthenium catalyst also comprises a gas diffusion substrate in which theruthenium catalyst is embedded, or wherein the anode structure comprisesa gas diffusion substrate on a surface of the layer comprising theruthenium catalyst opposite the electrocatalyst layer, or a combinationof these.
 4. A process for the preparation of an anode structureaccording to claim 3, said process comprising a step of applying theruthenium catalyst to the gas diffusion substrate.
 5. The processaccording to claim 4, wherein said process comprises the further step offiring said anode structure.
 6. The process according to claim 5,wherein said firing process is carried out at a temperature below 375°C.
 7. The process according to claim 6, wherein said firing process iscarried out at a temperature below 275° C.
 8. The process according toclaim 5, wherein said firing process is carried out in an environmentdevoid of oxygen.
 9. The process according to claim 8, wherein saidfiring process is carried out in nitrogen.
 10. A process for thepreparation of an anode structure according to claim 3, said processcomprising the steps of applying the ruthenium catalyst and theelectrocatalyst to the gas diffusion substrate.
 11. The anode structureaccording to claim 3, wherein the layer comprising the rutheniumcatalyst is on the surface of the electrocatalyst layer and alsocomprises said gas diffusion substrate In which the ruthenium catalystis embedded.
 12. The anode structure according to claim 3, wherein thelayer comprising the ruthenium catalyst is on the surface of theelectrocatalyst layer, and wherein the anode structure further comprisessaid gas diffusion substrate on said surface of the layer comprising theruthenium catalyst that is opposite the electrocatalyst layer.
 13. Theanode structure according to claim 3, wherein the layer comprising theruthenium catalyst is separated from said electrocatalyst layer by saidinterspersed gas diffusion substrate.
 14. The anode structure accordingto claim 1, wherein said anode structure prevents poisoning of theelectrocatalyst.
 15. A process for the preparation of an anode structureaccording to claim 1, said process comprising the steps of applying theruthenium catalyst and the electrocatalyst to the membrane.
 16. Amembrane electrode assembly comprising an anode structure according toclaim
 1. 17. A fuel cell comprising a membrane electrode assemblyaccording to claim 16.